Add docstrings to all unittest.TestCase:s (#4391)

Summary:
This avoids logging a long default docstring which makes the output of test collection a lot easier to read and thus debug.

I also updated the mv2net weight parameter as the current way of calling it is deprecated. This also removes a warning from test collection.

Change-Id: I05f000e9ef42ab9d63f234539da3309f69ccbe16

Pull Request resolved: https://github.com/pytorch/executorch/pull/4391

Reviewed By: cccclai

Differential Revision: D60404067

Pulled By: digantdesai

fbshipit-source-id: 4ad44eb35887278cacd7e2c0ac93114b125e72af